Take the red line nr. 13 till the end station, Ropsten. Don't go down the escalators, instead follow the signs to Hjorthagen/Spårvägsmuseet/Jägmästargatan and half harrows to Onn-In (Plan ca. 15 mins to Onn-in).
Alternatively, catch Bus nr. 6 / 75 to Ropsten, exit Storängsvägen OR Bus nr. 54 to Hjorthagen exit Jaktgatan (less walking).